This invention is related to ion sources that are suitable for use in ion implanters and, more particularly, to ion sources having indirectly heated cathodes.
An ion source is a critical component of an ion implanter. The ion source generates an ion beam which passes through the beamline of the ion implanter and is delivered to a semiconductor wafer. The ion source is required to generate a stable, well-defined beam for a variety of different ion species and extraction voltages. In a semiconductor production facility, the ion implanter, including the ion source, is required to operate for extended periods without the need for maintenance or repair.
Ion implanters have conventionally used ion sources with directly heated cathodes, wherein a filament for emitting electrons is mounted in the arc chamber of the ion source and is exposed to the highly corrosive plasma in the arc chamber. Such directly heated cathodes typically constitute a relatively small diameter wire filament and therefore degrade or fail in the corrosive environment of the arc chamber in a relatively short time. As a result, the lifetime of the directly heated cathode ion source is limited.
Indirectly heated cathode ion sources have been developed in order to improve ion source lifetimes in ion implanters. An indirectly heated cathode includes a relatively massive cathode which is heated by electron bombardment from a filament and emits electrons thermionically. The filament is isolated from the plasma in the arc chamber and thus has a long lifetime. Although the cathode is exposed to the corrosive environment of the arc chamber, its relatively massive structure ensures operation over an extended period.
The cathode in the indirectly heated cathode ion source must be electrically isolated from its surroundings, electrically connected to a power supply and thermally isolated from its surroundings to inhibit cooling which would cause it to stop emitting electrons. Known prior art indirectly heated cathode designs utilize a cathode in the form of a disk supported at its outer periphery by a thin wall tube of approximately the same diameter as the disk. The tube has a thin wall in order to reduce its cross sectional area and thereby reduce the conduction of heat away from the hot cathode. The thin tube typically has cutouts along its length to act as insulating breaks and to reduce the conduction of heat away from the cathode.
The tube used to support the cathode does not emit electrons, but has a large surface area, much of it at high temperature. This area loses heat by radiation, which is the primary way that the cathode loses heat. The large diameter of the tube increases the size and complexity of the structure used to clamp and connect to the cathode. One known cathode support includes three parts and requires threads to assemble.
The indirectly heated cathode ion source typically includes a filament power supply, a bias power supply and an arc power supply and requires a control system for regulating these power supplies. Prior art control systems for indirectly heated cathode ion sources regulate the supplies to achieve constant arc current. A difficulty in using a constant arc current system is that, if the beamline is tuned, beam current measured at the downstream end of the beamline can increase either due to the tuning, which increases the percent of current transmitted through the beamline, or due to an increase in the amount of current extracted from the source. Since beam current and transmission are influenced by the same plurality of variables, it difficult to tune for maximum beam current transmission.
A prior art approach that has been utilized in ion sources with directly heated cathodes is to control the source for constant extraction current rather than constant arc current. In all cases where the source is controlled for constant extraction current, the control system drives a Bernas type ion source where the cathode is a directly heated filament.
According to an aspect of the invention, an indirectly heated cathode ion source includes an arc chamber housing defining an arc chamber having an extraction aperture, an extraction electrode positioned outside of the arc chamber in front of the extraction aperture, an indirectly heated cathode positioned within the arc chamber, and a filament for heating the cathode. A filament power supply provides a current for heating the filament, a bias power supply provides a voltage between the filament and the cathode, an arc power supply provides a voltage between the cathode and the arc chamber housing, and an extraction power supply provides a voltage between the arc chamber housing and the extraction electrode, for extracting from the arc chamber an ion beam having a beam current. The ion source further includes an ion source controller for controlling the beam current extracted from the arc chamber at or near a reference extraction current. The ion source may also include an extraction current sensor for sensing an extraction power supply current that is representative of the extracted beam current and, in another embodiment, a suppression electrode positioned between the arc chamber housing and the extraction electrode and a suppression power supply coupled between the suppression electrode and ground.
The ion source controller may include feedback means for controlling the extracted beam current in response to an error value based on the difference between a sensed beam current and the reference extraction current. In one embodiment, the feedback means may include means for controlling a bias current supplied by the bias power supply in response to the error value. In another embodiment, the feedback means may include means for controlling a filament current supplied by the filament power supply in response to the error value. The feedback means may include a Proportional-Integral-Derivative controller. The indirectly heated cathode ion source, including a cathode and a filament for heating the cathode, may be controlled by sensing a beam current extracted from the ion source, and controlling a bias current between the filament and the cathode in response to an error value based on the difference between the sensed beam current and a reference extraction current.
In a first control algorithm, a beam current extracted from the ion source is sensed and a bias current between the filament and the cathode is controlled in response to an error value based on the difference between the sensed beam current and a reference extraction current. The algorithm may further include maintaining a filament current and an arc voltage at a constant value, and not regulating a filament voltage and an arc current.
In a second control algorithm, a beam current extracted from the ion source is sensed and a filament current through the filament is controlled in response to an error value based on the difference between the sensed beam current and a reference extraction current. The algorithm may further include maintaining a bias current and an arc voltage at a constant value, and not regulating a bias voltage and an arc current.
According to another aspect of the invention, a method for controlling an indirectly heated cathode ion source includes sensing a beam current extracted from the ion source, and controlling the beam current extracted from the ion source in response to an error value based on the difference between the sensed beam current and a reference extraction current. According to yet another aspect of the invention, a method for controlling a beam current extracted from an arc chamber includes providing an arc chamber housing defining an arc chamber having an extraction aperture; an extraction electrode positioned outside of the arc chamber in front of the extraction aperture; an indirectly heated cathode positioned within the arc chamber; a filament for heating the cathode; a filament power supply for providing current for heating the filament; a bias power supply coupled between the filament and the cathode; an arc power supply coupled between the cathode and the arc chamber housing; an extraction power supply, coupled between the arc chamber housing and the extraction electrode, for extracting from the arc chamber an ion beam having a beam current; and an ion source controller for controlling the beam current extracted from the arc chamber at or near a desired level, in response to an extraction current supplied by the extraction power supply.